The polyphenylene ethers are known and described in numerous publications, including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875; and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, all incorporated herein by reference. They are useful for many commercial applications requiring high temperature resistance and, because they are thermoplastic, they can be formed into films, fibers and molded articles. In spite of these desirable properties, parts molded from polyphenylene ethers are somewhat brittle due to poor impact strength. In addition, the relatively high melt viscosities and softening points are considered a disadvantage for many uses. Films and fibers can be formed from polyphenylene ethers on a commercial scale using solution techniques, but melt processing is commercially unattractive because of the required high temperatures needed to soften the polymer and the problems associated therewith such as instability and discoloration. Such techniques also require specially designed process equipment to operate at elevated temperatures. Molded articles can be formed by melt processing techniques, but, again, the high temperatures required are undesirable.
In addition, although the polyphenylene ether resins have outstanding hydrolytic stability, making them very useful in contact with aqueous media, e.g., in dishwasher and laundry equipment, they will soften or dissolve in contact with many aggressive solvents, e.g., halogenated or aromatic hydrocarbons and gasoline, which limits their use in automotive applications.
It is known in the art that the properties of the polyphenylene ethers can be materially altered by forming compositions with other polymers. For example, Finholt, U.S. Pat. No. 3,379,792, discloses that flow properties of polyphenylene ethers are improved by preparing a composition thereof with from about 0.1 to 25 parts by weight of a polyamide. In Gowan, U.S. Pat. No. 3,361,851, polyphenylene ethers are formed into compositions with polyolefins to improve impact strength and resistance to aggressive solvents. In Cizek, U.S. Pat. No. 3,383,435, incorporated herein by reference, there is provided a means to simultaneously improve the melt processability of the polyphenylene ethers and upgrade many properties of polystyrene resins. The Cizek patent disclosed that polyphenylene ethers and polystyrene resins, including many modified polystyrenes, are combinable in all proportions to provide compositions having many properties improved over those of either of the components.
Preferred embodiments of the Cizek patent are compositions comprising a rubber modified high-impact polystyrene and a poly-(2,6-dialkyl-1,4-phenylene)ether. Such compositions are important commercially because they provide both an improvement in the melt processability of the polyphenylene ether and an improvement in the impact resistance of parts molded from the compositions. Furthermore, such compositions of the polyphenylene ether and the rubber modified high-impact polystyrene may be custom formulated to provide pre-determined properties ranging between those of the polystyrene resin and those of the polyphenylene ether by controlling the ratio of the two polymers. The reason for this is that the Cizek compositions exhibit a single set of thermodynamic properties rather than the two distinct sets of properties i.e., one for each of the components of the composition, as is typical with compositions or blends of the prior art.
The preferred embodiment of the Cizek patent is disclosed to comprise poly(2,6-dimethyl-1,4-phenylene)ether and a rubber modified high-impact polystyrene (identified in Example 7 as Lustrex HT88-1 of Monsanto Chemical Company). It is known in the art that Monsanto HT-88 high impact polystyrene contains an elastomeric gel phase dispersed through a polystyrene matrix and that this elastomeric phase comprises about 20.7% by weight of the composition. In addition, it is known that in the gel free polystyrene matrix in Lustrex 88, the weight average molecular weight, M.sub.w, is about 251,000 and the number average molecular weight, M.sub.n, is about 73,000, and, therefore, the polydispersity, i.e., the ratio M.sub.w /M.sub.n is about 3.44. This is shown, for example, in Table 3 in Vol. 13, Encyclopedia of Polymer Science and Technology, 1970, p. 401 et seq. Thus the preferred embodiment of the Cizek patent, which was disclosed to have a notched Izod impact strength ranging from 1.0 to 1.5 ft.lbs./in. notch (Standard Method, ASTM- D-256) comprised a polyphenylene ether and a rubber modified high-impact polystyrene resin, the polystyrene in the matrix having a weight average molecular weight of about 251,000.
The Staudinger equation EQU [.eta.] = KM.sup.a
wherein [.eta.] is the intrinsic viscosity, K is a constant, M is the molecular weight (close to the weight average) and a is a constant depending on the system, is used by those skilled in the art to determine relative molecular weights in a given polymer system. For the purposes of this disclosure, the relative molecular weights of the matrix polystyrene will be discussed in terms of intrinsic viscosity. The intrinsic viscosity of a polymer solution is usually estimated by determining the specific viscosity at several low concentrations and extrapolating the values to zero concentration. Determined in known ways, the intrinsic viscosity of the matrix in Lustrex HT-88 is of the order of 0.8 deciliters/gram.
It is generally recognized that the properties of impact resistant polystyrenes are highly dependent on the number, size and character of dispersed elastomeric particles. Moreover, while most commercial impact polystyrenes contain from 3 to 10% by weight of a dispersed rubber phase comprising particles of polybutadiene or rubbery butadiene-styrene copolymer, the polystyrene in the matrix usually has a limited distribution of weight average molecular weights and, especially, the upper limit appears to be about 260,000 (the same as Lustrex-88 used in Cizek). By way of illustration, the four commercial products shown in the Encyclopedia of Polymer Science and Technology, Interscience, Vol. 13, p. 400 (1970), Table 3 have M.sub.w values of 209,000; 251,000; 252,000 and 164,000.
Moreover, as part of a study of the effect of the molecular weight distribution of the matrix polystyrene in high impact polystyrenes, Wagner et al., Encyclopedia of Rubber Technology, Vol. 43, 1970, p. 1136, stated as a generally recognized fact that commercial thermally initiated impact polystyrenes have a weight average molecular weight, M.sub.w, in the range of 250,000. Proceeding from this point, Wagner et al blended in increasing amounts of polystyrene of higher molecular weight, M.sub.w, 305,000 (intrinsic viscosity of about 0.90). In so doing, the rubber content was decreased, and there was found a gradual decrease in properties, particularly in impact strength.
These combined teachings indicate that increasing the molecular weight of the polystyrene in the matrix of the rubber modified polystyrene used in the Cizek embodiments would not increase the physical properties of the composition with polyphenylene ether resins, but would tend to cause them gradually to decrease.
In view of the above, it has now unexpectedly been found that compositions of a polyphenylene ether with a rubber modified polystyrene resin can be provided with substantially improved impact strengths if the polystyrene matrix phase has an intrinsic viscosity of at least 1.0 deciliters/gram measured in chloroform at 30.degree.C. Correspondingly, the weight average molecular weight, M.sub.w, is above 350,000, which is substantially above the 305,000 in the additive used by Wagner et al., and the range of 164,000-252,000 used in the prior art.
In addition, in such compositions, the surface appearance, especially gloss, is unexpectedly improved, as is the resistance to aggressive solvents, such as gasoline.
With respect to gasoline resistance, it is disclosed in the Cizek patent that this can be improved by using as the polystyrene resin component, in combination with the polyphenylene ether resin a copolymerized alkenyl cyanide, such as acrylonitrile, either in the rubber backbone or copolymerized with the styrene. Without such an expedient, the styrene resins used in the Cizek compositions, e.g., Styron-666 and Lustrex HT-88, provide molded parts which have poor gasoline resistance, precluding their use in many applications, particularly automotive uses.
It has also now been discovered that the gasoline resistance of polyphenylene ether resins and blends thereof with styrene resins is remarkably improved if the molecular weight of polystyrene in the matrix is significantly increased above that in the commercial products and those shown in the prior art. While the prior art compositions, which have polystyrene matrix intrinsic viscosities in the range 0.75-0.90, provide 40/60 and 40/65 compositions with poly(2,6-dimethyl-1,4-phenylene)ether that fail catastrophically in gasoline at 1% strain in 10-15 seconds, if the intrinsic viscosity is increased to above 1.0, in accordance with this invention, no failure, is seen even after 30 minutes.